Synthesis and characterization studies of new five member ring metal chelates derived from benzion phenoxyacetyl hydrazone(H₂BPAH)

2.1 Synthesis of the ligand

All chemicals used are BDH (British Drug LTD, England) quality. Benzion Phenoxyacetyl hydrazone structure I was prepared by refluxing equimolar amounts of benzoin (21.1 gm, 0.1 mol) and phenoxyacetyl hydrazine (16.6 gm, 0.1 mol) in 100 ml absolute ethanol for 0.5 h. Upon cooling white crystals were separated. The product was filtered, washed, recrystallized from absolute ethanol and finally dried in a desiccator over fused CaCl₂, structure I, yield 27.8 g, m.p 142 °C.

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2.2 Synthesis of metal chelates

The [Cd(H2BPAH)Cl₂] complex was prepared by mixing together an ethanolic solution (25 ml) of cadmium chloride (1 mmole) and H₂BPAH (0.36 g, 1 mmole) in the same solvent (25 ml). The reaction mixture was heated under reflux for 0.5 h. The [M(HBPAH)OAc] (M⚌Zn(II), Cd(II) and U(VI)O₂) and [M(HBPAH)(H₂O)₂](M⚌Co(II), Ni(II) and Cu(II)) were prepared by the same method using the corresponding metal acetate (1 mmole) in 25 ml water or ethanol. The [Pd(HBPAH)₂] complex was prepared by the same previous method using 2 mmole of H₂BPAH in 50 ml absolute ethanol and 1 mmole of Na₂PdCl₄ in 25 ml aqueous solution.

The resulting complexes during reflux or after cooling were filtered, washed several times with absolute ethanol or an aqueous ethanolic solution followed by diethylether and finally dried in a vacuum desiccator over fused CaCl₂.

2.3 Physical measurements

The metal and chloride contents were analyzed by standard method (Vogel, 1975). Carbon and hydrogen were carried out at the Microanalytical Unit at the Mansoura University. Magnetic moments at room temperature were determined using a Gouy balance and Hg[Co(SCN)₄] as a calibrant. IR spectra were recorded on a Mattson 5000 FTIR Spectrometer as KBr disks; electronic spectra in dimethylsulpfxide (DMSO) were obtained using an UV2–100 Unicam UV–visible spectrometer. ¹H NMR spectra were recorded on Prucker Ac 400 Spectrometer. Molar conductances of the solid complexes (10⁻³ M) in DMSO at room temperature were measured using a type CD6NGT Tacussel Conductivity Bridge. ESR spectra were obtained on a Bruker EMX spectrometer working in the X-Band (9.78 GHz) with 100 kHz modulation frequency.

2.4 Corrosion studies

2.5 Biological activity

The antimicrobial activity of the ligand and some complexes were tested as antibacterial agents. In this technique pores are made using a sterile cork paper in the solidified agar medium and aliquots were prepared by putting 25 ml of inoculated agar into 15 cm petri-dishes and allowing them to solidify. Cups were made to receive 25 ml of the solution and allowed to diffuse and incubate at 37 °C for 24 h. Inhibition zone was measured (Gerhardt, 1981) and compared with that of gentamicin solution (commercial antibiotic, Memphis Co., Egypt, 1000 μg mL⁻¹). The experiment control was DMSO.

3.1 IR and ¹H NMR spectral studies

The principal IR bands of H₂BPAH and its metal complexes are listed in Table 2. The IR spectrum of H₂BPAH shows two bands in high frequency regions 3331, 3298 cm⁻¹ attributed to the stretching vibrations of (OH) and (NH) groups, respectively. The strong bands at 1693 and the shoulder at 1650 cm⁻¹ are assigned to υ(C⚌O) (Geary, 1971) and υ(C⚌N) (Chundak et al., 1986), respectively. The two medium intensity bands at 1236 and 1136 cm⁻¹ are probably due to the stretching vibration of (C–O) (Aggarwal et al., 1976; Biradar et al., 1975) alcoholic and the bending vibration of (OH) groups (Bekheit, 2012), respectively. The appearance of weak intensity bands in the 2150–2070 and 1920–1900 cm⁻¹ region suggests intramolecular hydrogen bonding (O….H–N) (Bullock and Tajmir, 1978) as shown in structure 1. The IR spectrum of [Cd(H₂BPAH)Cl ₂] shows that H₂BPAH behaves as a neutral bidentate ligand, coordinating through the carbonyl oxygen (C⚌O) and azomethine (C⚌N) nitrogen forming a five membered ring including the cadmium atom. This mode of chelation (structure II) is supported by the following evidences: (i) both υ(C⚌O) and υ(C⚌N) shift to a lower wavenumber, (ii) The stretching vibration of the (OH) group appears as a strong band in the high frequency regions at 3632 cm⁻¹ indicating the non-involvement of the OH group in bonding (Abu El-Reash et al., 1988); and (iii) the appearance of new bands in the low frequency region at 544 and 514 cm⁻¹ assigned to υ(Cd–O)(Speca et al., 1974) and υ(Cd–N) (Beecroft et al., 1974), respectively. The IR spectrum data of [Pd(HBPAH)₂] show that H₂BPAH behaves as a mononegative bidentate ligand which coordinates via the azomethine nitrogen (C⚌N) and the oxygen atom of the deprotanted alcoholic forming a five membered ring including the palladium atom as shown in structure III. This mode of complexation is supported by the following evidences. (i) υ(C⚌N) shifts to a lower wavenumber, (ii) the disappearance of δ(OH), (iii) υ(C–O)(Lal et al., 2001) alcoholic shifts to a lower wavenumber indicating the probable participation of the oxygen in coordination, (iv) υ(C⚌O) appears in the same position at 1692 cm⁻¹ as observed in the free ligand, suggesting that the carbonyl oxygen (C⚌O) does not take part in coordination and (v) the appearance of new bands at 508 and 473 cm⁻¹ assigned to υ(Pd–O) (Speca et al., 1974) and υ(Pd–N) (Beecroft et al., 1974), respectively.

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Table 2 IR spectral data of H₂BPAH and its metal complexes.

Compound	υ(C = O)	υ(C = N)	ν ( C = N–N = C )	υ(C–O)Hydroxyl	υ(C–O)Carbonyle	δ(OH)	υ(M–O)	υ(M–N)
H2BPAH	1693	1650	–	1236	–	1136	–	–
[Cd(H2BPAH)Cl2]	1683	1638	–	1246	–	1133	544	514
[Pd(HBPAH)2]	1692	1629	–	1214	–	–	508	473
[Zn(HBPAH)OAc]	1690w	1635	–	1208	–	–	512	460
[Cd(HBPAH)OAc]	1690w	1637	–	1220	–	–	513	449
[UO2(HBPAH)OAc]	1685	1624	–	1222	–	–	513	471
[Co(BPAH)(H2O)2]	–	–	1575	1218	1256	–	516	459
[Ni(BPAH)(H2O)2]	–	–	1576	1220	1250	–	514	457
[Cu(BPAH)(H2O)2]	–	–	1542	1221	1256	–	520	508

In [M(HBPAH)OAc] (M⚌Zn(II), Cd(II) and U(VI)O₂), H₂BPAH is a mononegative tridentate, coordinating via the azomethine nitrogen (C⚌N), the carbonyl oxygen (C⚌O) and the oxygen atom of the deprotonated alcoholic group. This mode of coordination is confirmed by the following observations: (i) both υ(C⚌O) and υ(C⚌N) shift to a lower wavenumber and decreased in intensity in some cases, (ii) the disappearance of δ(OH), (iii) υ(C–O) (Lal et al., 2001) alcoholic also shifts to a lower wavenumber and (iv) the appearance of new bands at 512–513 and 449–471 cm⁻¹ regions assigned to υ(M–O) (Speca et al., 1974) and υ(M–N) (Beecroft et al., 1974), respectively.

One of the two donor oxygens must bridge to a second metal center in order for both metal ions to achieve coordination number six, similar to other workers (Iskander et al., 1980), it seems more likely that the deprotonated alcoholic oxygen is the bridging oxygen as shown in structure IV.

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Also, a dimeric structure is suggested for this mode of complexation as shown in structure V. H₂BPAH acts as a tridentate ligand for one metal center and one of the two oxygens probably bridges to a second metal ion.

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The ¹H NMR spectrum of H₂BPAH in DMSO-d6 (Fig. 1) shows multiplet signals in the δ 6.26–7.78 ppm range assigned to the aromatic protons. The two signals at δ 12.56 and δ 11.21 ppm are attributable to the two protons of (OH) and (NH) groups, respectively. The appearance of the signal due to the proton of (OH) at a high value downfield from TMS suggests the presence of intramolecular hydrogen bonding between the phenolic oxygen (OH) and azomethine, while the multiplet signals appearing at δ 2.51 ppm may be assigned to the protons of CH₂ group.

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Figure 1

¹H NMR spectrum of H₂BPAH.

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A strong evidence of the deprotonation of the hydroxyl group comes from the ¹H NMR spectrum of [Pd(HBPAH)₂] complex (Fig. 2), which shows the disappearance of the single observed at δ 12.56 ppm in the free ligand due to the proton of the OH group.

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Figure 2

¹H NMR spectrum of [Pd(HBPAH)₂].

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3.2 Magnetic and electronic spectral studies

The magnetic moments, electronic absorption bands in (DMSO) and ligand filed parameters of metal complexes are reported in Table 3.

The electronic spectrum of [Cu(HBPAH)OAc(H₂O)] shows a broad band at 15087 cm⁻¹ due to the ${}^2{\text{B}}_{2\text{g}}{\to }^2{\text{E}}_{\text{g}}$ transition in a tetragonally distorted octahedral configuration (Lever, 1984), the band at 28571 may be attributed to charge transfer. The magnetic moment value of Cu(II) complexes (1.97 BM) lies within the range of Cu(II) ions.

The electronic spectrum of [Co(BPAH)OAc(H₂O)] complex gives two bands at 17391 and 19230 cm⁻¹ attributed to ${}^4{\text{T}}_{1\text{g}}{\to }^4{\text{A}}_{2\text{g}}$ and ${}^4{\text{T}}_{1\text{g}}{\to }^4{\text{A}}_{1\text{g}}$ (P) transitions, respectively. The calculated D_q, B and β values (Table 3) lie in the range reported for octahedral structure (Lever, 1984; Sacconi, 1979).The magnetic moment value (5.1 BM) of the Co(II) lies in the usual range reported for octahedral structure.

The electronic spectrum of [Ni(BPAH)(H₂O)₃] complex (Fig. 3) shows two absorption bands at 17,094 and 21,002 cm⁻¹ which are assignable to ${}^3{\text{A}}_{2\text{g}}{\to }^3{\text{T}}_{1\text{g}}$ (F) (υ₂) and ${}^3{\text{A}}_{2\text{g}}{\to }^3{\text{T}}_{1\text{g}}$ (P) (υ₃) transitions, respectively in octahedral environments around nickel(II) ions. The calculated values of the ligand filled parameters (Table 3) lie in the range reported for octahedral structure. The magnetic moment value (2.8 BM) is consistent with those reported for octahedral geometry (Lever, 1984; Sacconi, 1979).

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Figure 3

Electronic spectra of [Ni(BPAH)(H₂O)₂].

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The [Pd(HBPAH)₂] complex is diamagnetic and its electronic spectrum shows two bands at 22,989 and 28,986 cm⁻¹ assignable to ${}^1{\text{A}}_{1\text{g}}{\to }^1{\text{B}}_{1\text{g}}$ and ${}^1{\text{A}}_{1\text{g}}{\to }^1{\text{E}}_{1\text{g}}$ , transition, respectively in square planar geometry (Bhave and Kharat, 1980). The band at 37,073 cm⁻¹ may be due to charge transfer.

Finally, The UV spectrum of the [UO₂(BPAH)(H₂O)₃] complex shows (Fig. 4) two bands at 22,222 and 28,571 cm⁻¹ assignable to ${}^1{\sum }_{\text{g}}^{+}{\to }^2{\pi }_{\text{u}}$ the transition of dioxuranium(VI) and charge transfer probably n → π^∗ transition, respectively(Singh et al., 1984).

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Figure 4

Electronic spectra of [UO₂(HBPAH)OAc].

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3.3 Studying the corrosion behavior

At present, weight-loss measurement is the most accurate and precise method for determining the metal corrosion rate because the experimentation is easy to replicate and, although long exposure times may be involved, the relatively simple procedure reduces the propensity to introduce systematic errors (Rabar and Shinde, 1983). The inhibition action of H₂BPAH on the dissolution of aluminum in 0.5 M HCl was investigated (Fig. 5). Aluminum was selected for this study due to its numerous industrial applications and consequently its corrosion inhibition in pickling baths is of great importance. The inhibition efficiency depends on the additive compounds and on many factors which include the number of adsorption active centers and their charge density, molecular size and the mode of interaction with metal surface (Zou et al., 2011). Weight loss determinations (Ghatlacharya and Bera, 1975) of aluminum in 0.5 M HCl after 30 min yielded convincing evidence for the application of H₂BPAH as active corrosion inhibitor. (Table 4) The maximum efficiency is 83.3% for the concentration of 5 × 10⁻³ mol L⁻¹ which indicates an excellent corrosion inhibition for aluminum (Fouda et al., 1986; ASTM, 1980; Rozenfeld, 1981). Also, tests for corrosion current gave result in the same way as weight loss does. Further studies on H₂BPAH will be conducted^.

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Figure 5

Weight loss-time curves for the corrosion of aluminum in 0.5 M HCl in the absence and presence of different concentrations of compound (H₂BPAH) at 30 °C.

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3.4 Antimicrobial activity

The antimicrobial activities of ligand and complexes against Bacillus thuringiensis and pseudomonas aureginosa are summarized in Table 5. Growth inhibition zones are proportional to the antimicrobial activity of the tested compound. The data suggest that cobalt(II) and nickel(II) complexes have similar antimicrobial activity (James and Akaranta, 2011; Gerhardt, 1981; Nutan et al., 2012; Petchiammal et al., 2012) against gram-positive and gram- negative bacteria.